Generally, power-conservation measures are required of battery operated devices. As an example, battery-operated ultrasonic flow meters are intermittently operated so far as the accuracy of the flow meters permits. Ultrasonic flow meters measure the velocity of a flow based on measurement of propagation time of ultrasonic waves, and calculate the quantity of flow by multiplying the flow rate with the cross-sectional area of the piping. Propagation time of ultrasonic waves is measured by using pulses of a fixed frequency and counting the number of pulses between transmission and reception of the ultrasonic waves, and multiplying the pulse frequency by the number of pulses. When making intermittent flow measurements, it is power-thrifty if these pulses can be stopped when flow rate measurement is stopped. For generating pulses of the fixed frequency to be used for propagation time measurement of ultrasonic waves, an oscillation circuit containing a ceramic oscillator or a crystal oscillator is used.
However, as these oscillators require some time after start of operation until generation of stable pulses, it is necessary to start the oscillation circuit in advance before starting flow rate measurement. Here, such start of the oscillation circuit is referred to as “cold start” for convenience' sake. The longer the period of cold start is, the more power conservation is disturbed.
Accordingly, there was proposed a method to advance stable operation of oscillation circuits by using an oscillation circuit and a ring oscillator, in which pulses adjusted to the oscillation frequency of the oscillator are generated by the ring oscillator, and stable operation of the oscillation circuit is advanced by giving these pulses to the basic oscillation circuit for a certain period of time from an early period of the start (ref. Patent Literature 1, for example). Here, the oscillation circuit comprises a crystal oscillator, a capacitor to insure reliability of oscillation, a resistor, and an amplifier. The ring oscillator has a current source having temperature correction capability.
A switching circuit lies between the basic oscillation circuit and the ring oscillator.
This method of advancing start of the oscillation circuit by giving a pulse to the oscillation circuit from outside at an early stage of start of the oscillation circuit is called “hot start” for convenience.
Though this is an effective method for advancing start of the oscillation circuit, it suffers a problem of calling for a ring oscillator having a power source with temperature-correction function, thus resulting in complicated circuit configuration.
There also was an ultrasonic flow meter that intermittently operated, for the purpose of reducing electric power consumption, a oscillator for making pulses for measuring transmission time of ultrasonic waves, and was equipped with oscillation initiation means for advancing vibration of the oscillator (ref. Patent Literature 2, for example). In Patent Literature 2, it is described that the method described in Patent Literature 1 is effective for intermittent operation of pulses to be used for measurement of transmission time of ultrasonic waves.
However, in the above configuration, as the start of the oscillation circuit is advanced in an early stage of the start of the oscillation circuit by giving a pulse from outside to the oscillation circuit, a current source having temperature correction function such as a ring oscillator is required thus resulting in a complicated circuit construction.    [Patent literature 1] U.S. Pat. No. 6,819,195    [Patent literature 2] International Publication No. 2008/081,610